The present invention relates to cyclotrons which are devices for accelerating a beam of ionized particles around a substantially spiral path lying normal to an axial magnetic field, so as to produce a continuous output beam of particles at the high energy levels required for research and other purposes involving ion bombardment.
In a cyclotron a beam of ionized particles travels past electrodes which are paired to have opposing electrical voltages applied to them. With each transition of the ionized particles past the differential voltage of a pair of electrodes, the particles gain energy. The voltages applied to the electrodes are alternating voltages of radio frequency and are applied at a frequency to synchronise with the transitions of the ionized particles. By causing the ionized particles to travel in a roughly circular path which lies normal to an axial magnetic field, the particles can be made to make numerous transitions past a small number of electrode pairs receiving acceleration and gaining in radius at each transition.
There is problem with the classical designs of cyclotrons caused by relativistic effects which increase the mass of the ion particles causing them to move more slowly and lose synchronism with the radio frequency energisation being applied to the electrodes.
The particles can be brought back up to the required speed and thus kept in synchronism with the radio frequency energisation by increasing the strength of the magnetic field with radius. The magnetic field is then said to have an isochronous field shape. This field shape does, however, cause a loss of focussing of the beam of ionized particles and to re-focus the particles as azimuthal variation or `flutter` is incorporated into the magnetic field.
Cyclotrons are usually constructed using resistive magnet coils enclosed within ferro-magnetic yokes, but the bulk and weight of these magnets has limited their use to establishments with large premises to house them. In addition, the amount of energy required to power the resistive coils puts a substantial demand on the electrical supply.
Designs of cyclotrons have been proposed using superconducting magnets but these have used ferro-magnetic yokes and have been fairly substantial in bulk and weight.